The present invention generally relates to radiation emitter devices such as, for example, light emitting diode (LED) packages, to methods of making radiation emitter devices, and to opto-electronic emitter assemblies incorporating optical radiation emitter devices.
As used herein, the term xe2x80x9cdiscrete opto-electronic emitter assemblyxe2x80x9d means packaged radiation emitter devices that emit ultraviolet (UV), visible, or infrared (IR) radiation upon application of electrical power. Such discrete opto-electronic emitter assemblies include one or more radiation emitters. Radiation emitters, particularly optical radiation emitters, are used in a wide variety of commercial and industrial products and systems, and accordingly come in many forms and packages. As used herein, the term xe2x80x9coptical radiation emitterxe2x80x9d includes all emitter devices that emit visible light, near IR radiation, and UV radiation. Such optical radiation emitters may be photoluminescent, electroluminescent, or another type of solid state emitter. Photoluminescent sources include phosphorescent and fluorescent sources. Fluorescent sources include phosphors and fluorescent dyes, pigments, crystals, substrates, coatings, and other materials.
Electroluminescent sources include semiconductor optical radiation emitters and other devices that emit optical radiation in response to electrical excitation. Semiconductor optical radiation emitters include light emitting diode (LED) chips, light emitting polymers (LEPs), organic light emitting devices (OLEDs), polymer light emitting devices (PLEDs), etc.
Semiconductor optical emitter components, particularly LED devices, have become commonplace in a wide variety of consumer and industrial opto-electronic applications. Other types of semiconductor optical emitter components, including OLEDs, LEPs, and the like, may also be packaged in discrete components suitable as substitutes for conventional inorganic LEDs in many of these applications.
Visible LED components of all colors are used alone or in small clusters as status indicators on such products as computer monitors, coffee makers, stereo receivers, CD players, VCRs, and the like. Such indicators are also found in a diversity of systems such as instrument panels in aircraft, trains, ships, cars, trucks, minivans and sport utility vehicles, etc. Addressable arrays containing hundreds or thousands of visible LED components are found in moving-message displays such as those found in many airports and stock market trading centers and also as high brightness large-area outdoor television screens found in many sports complexes and in some urban billboards.
Amber, red, and red-orange emitting visible LEDs are used in arrays of up to 100 components in visual signaling systems such as vehicle center high mounted stop lamps (CHMSLs), brake lamps, exterior turn signals and hazard flashers, exterior signaling mirrors, and for roadway construction hazard markers. Amber, red, and blue-green emitting visible LEDs are increasingly being used in much larger arrays of up to 400 components as stop/slow/go lights at intersections in urban and suburban intersections.
Multi-color combinations of pluralities of visible colored LEDs are being used as the source of projected white light for illumination in binary-complementary and ternary RGB illuminators. Such illuminators are useful as vehicle or aircraft maplights, for example, or as vehicle or aircraft reading or courtesy lights, cargo lights, license plate illuminators, backup lights, and exterior mirror puddle lights. Other pertinent uses include portable flashlights and other illuminator applications where rugged, compact, lightweight, high efficiency, long-life, low voltage sources of white illumination are needed. Phosphor-enhanced xe2x80x9cwhitexe2x80x9d LEDs may also be used in some of these instances as illuminators.
IR emitting LEDs are being used for remote control and communication in such devices as VCR, TV, CD and other audio-visual remote control units. Similarly, high intensity IR-emitting LEDs are being used for communication between IRDA devices such as desktop, laptop, and palmtop computers; PDAs (personal digital assistants); and computer peripherals such as printers, network adapters, pointing devices (xe2x80x9cmice,xe2x80x9d trackballs, etc.), keyboards and other computers. IR LED emitters and IR receivers also serve as sensors for proximity or presence in industrial control systems, for location or orientation within such opto-electronic devices such as pointing devices and optical encoders, and as read heads in such systems as barcode scanners. IR LED emitters may also be used in a night vision system for automobiles.
Blue, violet, and UV emitting LEDs and LED lasers are being used extensively for data storage and retrieval applications such as reading and writing to high-density optical storage disks.
For discrete LED devices and other discrete (xe2x80x9cpackagedxe2x80x9d) opto-electronic emitters, increased performance is dependent substantially upon increased reliable package power capacity, reduced package thermal resistance, and reduced susceptibility of the package to damage during auto-insertion, soldering and other circuit or system manufacturing operations.
Keeping discrete opto-electronic emitters cool during operation enhances performance in several ways. The efficiency of the emitter usually decreases in relation to increased operating temperature and increases in relation to reduced operating temperature. Conversely, emitter efficiency typically increases in relation to reduced internal operating temperature. The reliability of the emitter and life of the materials and sub-components comprising it usually improves in relation to decreased operating temperature. The consistency of the emitter""s emission spectra is usually improved in relation to decreased or more consistent operating temperature. The decay life of the emitter is usually improved in relation to reduced operating temperature. For these and other reasons, it is clearly beneficial to employ novel mechanisms for reducing the operating temperature of discrete opto-electronic emitters.
While the ambient environmental temperature is an external factor that cannot always be controlled, the temperature rise of the device above the ambient temperature is determined mainly by the device""s thermal resistance and operating power.
Unfortunately, most discrete opto-electronic emitters exhibit a characteristic contravening to the goal of reduced internal operating temperature. In short, these types of devices usually emit greater amounts of useful radiation in proportion to increased power up to some practical limit of the package or constituent materials or subcomponents. Thus, for applications where more radiation is useful (i.e., almost all applications known), it is beneficial to drive the device at the highest power consistent with device and system reliability and consistent with the power-radiation characteristics of the device. However, increased power in devices with finite (positive, non-zero) thermal resistance results in elevated internal operating temperatures.
It would be advantageous then to reduce internal operating temperature without having to reduce device power, or alternately to maintain internal operating temperature while increasing device power. This can be accomplished by reducing the device thermal resistance.
Billions of LED components are used in applications such as those cited above, in part because relatively few standardized LED configurations prevail and due to the fact that these configurations are readily processed by the automated processing equipment used almost universally by the world""s electronic assembly industries. Automated processing via mainstream equipment and procedures contributes to low capital cost, low defect rates, low labor cost, high throughput, high precision, high repeatability and flexible manufacturing practices. Without these attributes, the use of LEDs becomes cost prohibitive or otherwise unattractive from a quality standpoint for most high-volume applications.
Two of the most important steps in modern electronic assembly processes are high-speed automated insertion and mass-automated soldering. Compatibility with automatic insertion or placement machines and one or more common mass-soldering process are critical to large-scale commercial viability of discrete semiconductor optical emitters (including LEDs).
Thus, the vast majority of LEDs used take the form of discrete-packaged THD (Through Hole Device) or SMD (Surface Mount Device) components. These configurations primarily include radial-lead THD configurations known as xe2x80x9c5 mm,xe2x80x9d xe2x80x9cT-1,xe2x80x9d and xe2x80x9cT-1xc2xexe2x80x9d or similar devices with rectangular shapes, all of which are readily adapted onto tape-and-reel, tape-and-ammo, or other standardized packaging for convenient shipment, handling, and high-speed automated insertion into printed circuit boards on radial inserters. Other common discrete THD LED packages include axial components such as the xe2x80x9cpolyLED,xe2x80x9d which are readily adapted onto tape and reel for convenient shipment, handling, and high-speed automated insertion into printed circuit boards on axial inserters. Common SMD LED components such as the xe2x80x9cTOPLED(copyright)xe2x80x9d and Pixar are similarly popular as they are readily adapted into blister-pack reels for convenient shipment, handling, and high-speed automated placement onto printed circuit boards with chip shooters.
Soldering is a process central to the manufacture of most conventional circuit assemblies using standardized discrete electronic devices, whether THD or SMD. By soldering the leads or contacts of a discrete electronic component such as an LED to a printed circuit board, the component becomes electrically connected to electrically conductive traces on the PCB and also to other proximal or remote electronic devices used for supplying power to, controlling or otherwise interacting electronically with, the discrete electronic device. Soldering is generally accomplished by wave solder, IR reflow solder, convective IR reflow solder, vapor phase reflow solder, or hand soldering. Each of these approaches differ from one another, but they all produce substantially the same end effectxe2x80x94inexpensive electrical connection of discrete electronic devices to a printed circuit board by virtue of a metallic or inter-metallic bond. Wave and reflow solder processes are known for their ability to solder a huge number of discrete devices en masse, achieving very high throughput and low cost, along with superior solder bond quality and consistency.
Widely available cost-effective alternatives to wave solder and reflow solder processes for mass production do not presently exist. Hand soldering suffers from inconsistency and high cost. Mechanical connection schemes are expensive, bulky, and generally ill-suited for large numbers of electrical connections in many circuits. Conductive adhesives such as silver-laden epoxies may be used to establish electrical connections on some circuit assemblies, but these materials are more costly and expensive to apply than solder. Spot soldering with lasers and other selective-solder techniques are highly specialized for specific configurations and applications and may disrupt flexible manufacturing procedures preferred in automated electronic circuit assembly operations. Thus, compatibility with wave solder or reflow solder processes are desirable properties of a semiconductor optical emitter component. The impact of this property is far reaching, because these solder operations can introduce large thermal stresses into an electronic component sufficient to degrade or destroy the component. Thus an effective semiconductor optical emitter component must be constructed in such a fashion as to protect the device""s encapsulation and encapsulated wire bonds, die attach and chip from transient heat exposure during soldering.
Conventional solder processes require that the ends of the leads of the device (below any standoff or at a point where the leads touch designated pads on the PCB) be heated to the melting point of the solder for a sustained period. This profile can include temperature excursions at the device leads as high as 230-300 degrees C. for as long as 15 seconds. Given that the leads of the device are normally constructed of plated metals or alloys such as copper or steel, this high temperature transient poses no problems for the leads themselves. The problem instead is the ability of these leads to conduct heat along their length into the encapsulated body of the device. Since these heated leads are in contact with the interior of the body of the device, they temporarily raise the local internal temperature of the device during solder processing. This can harm the somewhat delicate encapsulation, encapsulated wire bonds, die attach and chip. This phenomenon represents one of the fundamental limitations of low-cost, opto-electronic semiconductor devices today.
Keeping the body of an electronic component from rising excessively above the glass transition temperature of its encapsulating material during solder processing is critical, since the Coefficient of Thermal Expansion of polymer encapsulating materials rises dramatically above their glass transition points, typically by a factor of two or more. Polymers will increasingly soften, expand and plastically deform above their glass transition points. This deformation from polymer phase transition and thermal expansion in encapsulants can generate mechanical stress and cumulative fatigue severe enough to damage a discrete semiconductor device, resulting in poor performance of the device and a latent predisposition toward premature field failure. Such damage typically consists of: 1) fatigue or fracture of electrical wire bonds (at the chip bond pads or at the lead-frame); 2) partial delamination or decomposition of die-attach adhesive; 3) micro-fracture of the chip itself; and 4) degradation of the device encapsulant, especially near the entry points of the leads into the encapsulant, and a compromised ability to seal out environmental water vapor, oxygen, or other damaging agents.
With regard to such thermal vulnerability, a crucial difference must be recognized between encapsulating materials suitable for non-optical electronic devices and those suitable for optical devices. The encapsulants used for non-optical devices may be opaque, whereas those used in constructing opto-electronic emitters and receivers must be substantially transparent in the operating wavelength band of the device. The side effects of this distinction are subtle and far ranging.
Since there is no need for transparency in non-optical devices, encapsulating materials for non-optical semiconductor devices may include a wide range of compositions containing a variety of opaque polymer binders, cross-linking agents, fillers, stabilizers and the like. Compositions of this type, such as heavily filled epoxy, may possess high glass transition temperatures (Tg), low thermal expansion coefficients (Cte), and/or elevated thermal conductivity such that they are suitable for transient exposures up to 175 degrees C. Opaque ceramic compositions may be thermally stable up to several hundred degrees C., with no significant phase transition temperatures to worry about, extremely low Cte and elevated thermal conductivity. For these reasons, exposure of conventional, opaque encapsulation materials for non-optical devices to electrical leads heated to 130 degrees C. or more for 10 seconds or so (by a solder wave at 230-300 degrees C.) is not normally a problem.
However, the need for optical transparency in encapsulants for opto-electronic emitters and receivers obviates use of most high-performance polymer-filler blends, ceramics and composites that are suitable for non-optical semiconductors. Without the presence of inorganic fillers, cross-linking agents or other opaque additives, the clear polymer materials used to encapsulate most opto-electronic devices are varieties of epoxies having relatively low Tg values, greater Cte, and low thermal conductivity. As such, they are not suitable for exposure to transient temperature extremes greater than about 130 degrees C., such as occurs during normal soldering.
To compensate for the potentially severe effects of damage from solder processing, prior art opto-electronic devices have undertaken a variety of improvements and compromises. The most notable improvement has been the relatively recent introduction of clear epoxies for encapsulation capable of enduring temperatures 10 to 20 degrees C. higher than those previously available (up to 130 degrees C. now versus the previous 110 degrees C.). While useful, this has only partially alleviated the problems notedxe2x80x94the newest materials in use still fall 50 degrees C. or more short of parity with conventional non-optical semiconductor encapsulation materials.
The most common compromise used to get around the transient temperature rise problem associated with soldering is to simply increase the thermal resistance of the electrical leads used in the device construction. By increasing the thermal resistance of these solderable leads, the heat transient experienced within the device body during soldering is minimized. Such an increase in thermal resistance can typically be accomplished in the following manner without appreciably affecting the electrical performance of the leads: 1) using a lead material with lower thermal conductivity (such as steel); 2) increasing the stand-off length of the leads (distance between solder contact and the device body); or 3) decreasing the cross-sectional area of the leads.
Using these three techniques, prior art devices have been implemented with elevated thermal resistance of the electrical leads to provide the desired protection from the solder process.
While effective at protecting prior art devices from thermal transients associated with soldering, there are limits to this approach, particularly in the application of high power semiconductor opto-electronic emitters. Increased lead thermal resistance results in elevated internal operating temperatures in prior art devices, severely compromising operational performance and reliability of these devices. The soldered electrical leads of most prior art LED devices conduct power to the device and serve as the primary thermal dissipation path for heat created within the device during operation. Thus the electrical leads in prior art devices must be configured to possess thermal resistance as low as possible to facilitate heat extraction during normal operation. Radiation and natural convection from prior art devices play only a minor role in transferring internal heat to ambient, and thermal conduction through their encapsulating media is severely impeded by the low thermal conductivity of the optical materials used. Therefore, the electrically and thermally conductive metal leads must extract a majority of the heat to ambient by the mechanism of conduction. Greater thermal resistance in the solderable pins of these devices, necessary to protect the device from the transient thermal effects of soldering operations, therefore causes a higher internal temperature rise within the encapsulated device body during operation.
The maximum temperature rise of a portion of the device body in contact with the semiconductor emitter under steady state is approximately equal to the product of the power dissipation of the emitter and the thermal resistance between the emitter and the ambient environment.
As previously discussed, severe consequences will result if the device internal temperature rises substantially above the encapsulant Tg value. Above this temperature, the Cte of the encapsulant typically increases very rapidly, producing great thermo-mechanical stress and cumulative fatigue at the LED wire bond and die attach. For most mobile applications such as automobiles, aircraft and the like, ambient temperatures commonly reach 80 degrees C. With encapsulation maximum operating temperatures in the range of 130 degrees C., an opto-electronic emitter for these applications must therefore limit its operational xcex94T to an absolute maximum of about 50 degrees C. This limits the power that can be dissipated in a given component, and in turn limits the current that may be passed through the component. Since the emitted flux of semiconductor optical emitters are typically proportional to the electrical current passed through them, limitations upon maximum electrical current also create limitations on flux generated.
Thus, it would be advantageous to reduce internal operating temperature without having to reduce device power, or alternately to maintain internal operating temperature while increasing device power by means of reducing the device thermal resistance without increasing device vulnerability to transient thermal processing damage from soldering.
Other prior art devices have avoided these constraints, but have achieved high performance only by ignoring the needs of standardized, automated electronic assembly operations and adopting configurations incompatible with these processes. Still other prior art devices have achieved high performance by employing unusually expensive materials, sub-components, or processes in their own construction.
For example, one prior art approach that has been used to overcome these limitations uses hermetic semiconductor packaging, hybrid chip-on-board techniques, exotic materials such as ceramics, KOVAR and glass, or complex assemblies instead of or in addition to polymer encapsulation. While relevant for certain high-cost aerospace and telecommunications applications (where component cost is not a significant concern), such devices require expensive materials and unusual assembly processes. This results in high cost and restricted manufacturing capacityxe2x80x94both of which effectively preclude the use of such components in mass-market applications. The devices disclosed in U.S. Pat. No. 4,267,559 issued to Johnson et al. and U.S. Pat. No. 4,125,777 issued to O""Brien et al. illustrate good examples of this.
The Johnson et al. patent discloses a device which includes both a TO-18 header component and a heat coupling means for mounting an LED chip thereto and transferring internally generated heat to external heat dissipating means. The header consists of several components, including a KOVAR member, insulator sleeves and electrical posts, and is manufactured in a specialized process to ensure that the posts are electrically insulated as they pass through the header. The heat coupling means is a separate component from the header and is composed of copper, copper alloys, aluminum or other high thermal conductivity materials. According to the teachings of Johnson et al., the KOVAR header subassembly and copper heat coupling means must be bonded together with solder or electrically conductive adhesive for electrical continuity, allowing flow of electrical current into the heat coupling means and subsequently into the LED chip. Furthermore, the header and heat coupling means of the Johnson et al. patent are made of completely dissimilar materials and must be so because of their unique roles in the described assembly. The header must be made of KOVAR in order that it may have a similar coefficient of thermal expansion to the insulator sleeves that run through it. At least one such sleeve is necessary to electrically isolate electrical pins from the header itself. However, KOVAR has relatively low thermal conductivity, necessitating the inclusion of a separate heat coupling means made of a material such as copper with a higher thermal conductivity. Since the header is a complex subassembly itself and is made of different materials than the heat coupling means, it must be made separately from the heat coupling means and then later attached to the heat coupling means with solder or an electrically conductive adhesive.
LED devices made similarly to the teachings of the Johnson et al. patent are currently being marketed in specialized forms similar to a TO-66 package. These devices are complex and typically involve insulated pin and header construction and/or include specialty sub-components such as ceramic isolation sheets within them.
Another approach which has been used to avoid damage to opto-electronic emitters from soldering has been to prohibit soldering of the component altogether or to otherwise require use of laser spot soldering or other unusual electrical attachment method. This can allow construction of a device with low thermal resistance from the semiconductor emitter within to the electrical pins without danger of device damage from soldering operations. The SnapLED 70 and SnapLED 150 devices made by Hewlett Packard illustrate this approach. In these devices, electrical connections are made to circuitry by mechanically stamping the leads to a simple metal circuit rather than soldering. The resultant devices are capable of continuous power dissipation as high as 475 mW at room temperature. This configuration, however, may complicate integration of such components with electronic circuits having higher complexityxe2x80x94such circuits are conventionally made using printed circuit boards, automated insertion equipment, and wave or reflow solder operations.
A final approach is illustrated by an LED package called the SuperFlux package (also known as the xe2x80x9cPiranhaxe2x80x9d), available from Hewlett Packard. The SuperFlux device combines moderate thermal resistance between the encapsulated chip and the solder standoff on the pins with a high-grade optical encapsulant and specialized chip materials and optical design. It achieves a moderate power dissipation capability without resorting to a non-solderable configuration such as the SnapLED. However, there are several significant problems with this configuration that inhibit its broader use.
The package geometry of the SuperFlux package renders it incompatible with conventional high-speed THD radial or axial insertion machinery or by SMT chip shooters known to the present inventors. Instead, it must be either hand-placed or placed by expensive, slow, robotic odd-form insertion equipment. The SuperFlux package geometry is configured for use as an xe2x80x9cend-onxe2x80x9d source onlyxe2x80x94no readily apparent convenient lead-bend technique can convert this device into a 90-degree xe2x80x9cside-lookerxe2x80x9d source. The moderate thermal resistance of the solderable pins of this device and relatively low heat capacity may leave it vulnerable to damage from poorly controlled solder processes. It may be inconvenient or costly for some electronic circuit manufacturers to control their soldering operations to the degree needed for this configuration. Finally, there is no convenient mechanism known to the inventors to outfit a SuperFlux package with a conventional active or passive heat sink.
A principle factor impeding further application of these and other LED devices in signaling, illumination and display applications is that there is not currently available a device that has a high power capability with high emitted flux where the device is easily adaptable to automated insertion and/or mass-soldering processes. These limitations have either impeded the practical use of LEDs in many applications requiring high flux emission, or they have mandated the use of arrays of many LED components to achieve desired flux emission.
Conventional xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d devices have a high thermal resistance, typically in excess of 240 degrees C. per watt and usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely or cyclically above 130 degrees C. (less for any but the best materials clear available). With typical ambient temperatures commonly exceeding 85 degrees C. in the automotive environment, the temperature rise in these devices must be limited to 45 degrees C. in order to properly avoid these material limits. This means that the device power must be limited to approximately 0.18 W. With a reasonable design tolerance of 33 percent to accommodate manufacturing variances, the practical reliable power limit of this device must be approximately 0.12 W. This is not a lot of power, and the emitted flux of these devices is thus limited. To overcome this, many of these devices are often used in combination to produce the luminous or radiant flux needed for an application (e.g., up to 50 for an automotive CHSML, up to 400 for a traffic signal lamp).
Hewlett Packard""s SuperFlux or Piranha devices have a lower thermal resistance than xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d devices, typically around 145 degrees C. per watt. As with xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d devices, SuperFlux or Piranha devices usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely, or cyclically above 130 degrees C. (less for any but the best materials clear available). With typical ambient temperatures commonly exceeding 85 degrees C. in the automotive environment, the temperature rise in these devices must be limited to 45 degrees C. in order to properly avoid these material limits. This means that the device power must be limited to approximately 0.3 W. Because these devices are attached subsequently with thermally stressful wave or other solder operations, and because their thermal resistance from lead to junction is reduced, they are more susceptible to damage during processing into circuits. Thus, a higher design tolerance of 40 percent should be used to accommodate manufacturing variances and increased susceptibility, and the practical reliable power limit of this device must be approximately 0.18 W. This is a substantial increase (33 percent) compared to xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d devices, it still is not a lot of power and the emitted flux of these devices is thus also limited. To overcome this, many of these devices are often used in combination to produce the luminous or radiant flux needed for an application (e.g., up to 30 for an automotive CHSML).
Hewlett Packard""s SnapLED devices have a lower thermal resistance than xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d or SuperFlux or Piranha devices, as low as 100 degrees C. per watt. As with xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d or SuperFlux, Piranha, or SnapLED devices usually are limited by clear encapsulation materials that lead to unreliability if the emitter in the device is operated continuously, routinely, or cyclically above 130 degrees C. (less for any but the best materials clear available). With typical ambient temperatures commonly exceeding 85 degrees C. in the automotive environment, the temperature rise in these devices must be limited to 45 degrees C. in order to properly avoid these material limits. This means that the device power must be limited to approximately 0.45 W. As noted above, because the thermal resistance of these devices from lead to junction is so low, they cannot be soldered by conventional means without being damaged. This severely limits their utility, but they still are suitable for some applications. Because these devices are attached subsequently with mechanically stressful clinching operations, they remain susceptible to damage during processing operations. Thus, a higher design tolerance of 40 percent should be used to accommodate manufacturing variances and potentially increased processing damage susceptibility, and the practical reliable power limit of this device must be approximately 0.27 W. This is a significant increase compared to xe2x80x9c5 mmxe2x80x9d or xe2x80x9cT 1xc2xexe2x80x9d or SuperFlux or Piranha devices, but it still is not a lot of power (and is achieved at a sacrifice in conventional solderability). To overcome the resulting limited flux from these devices, many are often used in combination to produce the luminous or radiant flux needed for an application (e.g., up to 12 for an automotive CHSML and up to 70 for an automotive rear combination stop/turn/tail lamp).
Surface mount devices such as the TOPLED(copyright), PLCC and Hewlett Packard""s xe2x80x9cHigh-Fluxxe2x80x9d or xe2x80x9cBarracudaxe2x80x9d devices use dissimilar polymer materials in their construction, the first in order of assembly being a plastic material that forms the basic structure of the device body and holds the device leads together. However, this approach requires that the lead frames be processed initially via insert molding (to emplace the first supporting material around the lead frame), then die mounting, then wire bonding and then a second stage of molding. The second stage of molding must be the optical molding (to first provide an opportunity for die bonding and wire bonding). Such a design and process are difficult and expensive to execute with high yield and high quality. Accumulated variances would be excessive from the multistage molding scheme, interrupted by die and wire bonding.
An additional problem faced by designers of conventional LED devices is that the wire bond used to join one of the LED leads to the LED chip can break or lose contact with the lead or the chip. Such failure can occur, for example, due to shear forces that are transferred to the wire bond through the encapsulant or thermal expansion/contraction of the encapsulant around the wire bond.
The other forms of radiation emitters mentioned above also experience performance degradation, damage, increased failure probability, or accelerated decay if exposed to excessive operating temperatures.
Consequently, it is desirable to provide a radiation emitter device that has the capacity for higher emission output than conventional LED devices while being less susceptible to failure due to a break in the wire bond contact or other defect that may be caused by excessive operating temperatures.
Additionally, it is desirable to provide a radiation emitter device having improved emission output over that of conventional LED devices while retaining the same size and shape of the conventional LED devices so as to facilitate the immediate use of the inventive LED devices in place of the conventional LED devices while also requiring minimal modification to the apparatuses that are used to manufacture the LED devices.
Accordingly, it is an aspect of the present invention to provide a radiation emitter device that overcomes the above-noted problems and that provides improved performance and less vulnerability to fatal damage. According to one embodiment of the present invention, a radiation emitting device comprises at least one radiation emitter, first and second electrical leads electrically coupled to the radiation emitter, and an integral encapsulant configured to encapsulate the radiation emitter and a portion of the first and second electrical leads. The encapsulant has at least a first zone and a second zone. The second zone exhibits at least one different characteristic from the first zone. The different characteristic may be a physical, structural, and/or compositional characteristic. For example, the at least one different characteristic may include at least one or more of the following: mechanical strength, thermal conductivity, thermal capacity, specific heat, coefficient of thermal expansion, adhesion, oxygen impermeability, moisture impermeability, and transmittance for radiation emitted from the radiation emitter.
A method of making a radiation emitting device according to the present invention comprises the steps of (1) attaching and electrically coupling at least one radiation emitter to a lead frame to form a subassembly; (2) inserting the subassembly into a mold cavity; (3) partially filling the mold cavity with a first encapsulant material; (4) filling the remainder of the mold cavity with a second encapsulant material; and (5) removing the encapsulated subassembly from the mold cavity.
For a wide range of otherwise-conventional discrete opto-electronic emitters, the present invention accomplishes a significant increase in the reliable package power capacity and accomplishes significant reductions in the package thermal resistance and package damage susceptibility in novel ways.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.